Dehydrofluorination of polyfluoroalkanes



Patented June 8, 1948 DEHYDROFLUORINATION OFPOLY- FLUOROALKANES OliverWilfred Cass, Niagara Falls, N. Y., assignor to E. I. du Pont de Nemonrs& Company, Wilmington, Del., a corporation of Delaware No Drawing.Application September 5, 1946, Z

- Serial No. 695,049

6 Claims. (01. 260653) 1 This invention relates to the catalyticdehydrofiuorination of polyfluoroalkanes and, more particularly, to thedehydrofluorination of 1,1-difluoroethane to obtain vinyl fluoride.

The dehydrofluorination of polyfluoroalkanes to obtain fluoroalkenes hasbeen accomplished by pyrolysis both in the absence and presence of.

various metals and salts as supported catalysts. In general,temperatures upwards of 600 C. are required and, as a consequence ofthese high temperatures, solid or tar-like carbonaceous deposits maycollect on the catalyst, resulting in catalyst deactivation and/orplugging of the reactor. Further, at higher temperatures there is atendency for the desired fiuoroalkenes to react to give acetyleniccompounds. Thus, for example, in the dehydrofluorination of1,1-difluoroethane, not

only vinyl fluoride but also appreciable amounts of acetylene may beformed.

An object of the present invention is to provide an improved process ofdehydrofluorinating polyfiuoroalkanes to obtain fluoroalkenes. A furtherobject is to provide such a process in which undesired secondaryreactions are avoided. A more particular object is to provide a processof dehydrofiuorinating 1,1-difiuoroethane to obtain vinyl fluoride atrelatively low temperatures with the almost complete suppression of thereaction wherein acetylene is formed. Other objects will be apparentfrom the description of the invention iven hereinafter.

The above objects are accomplished according to the present invention byheating in the vapor state a polyfluoroalkane containing 2 to 4 carbonatoms, inclusive, with at least two fluorine atoms attached to onecarbon atom and at least one hydrogen atom on an adjacent carbon atom,in the presence of 0.1% to by volume of oxygen at a temperature of 250C. to 700 C. Preferably, the temperature is kept between 300 C. and 600C., the proportion of oxygen is between 1 and 4% by volume of themixture of oxygen and the polyfiuoralkane, and the contact time at thetemperature indicated is less than three minutes.

A convenient method for carrying out the present invention is to pass amixture of the polyfluoroalkane and oxygen, such mixture containingabout 2% oxygen by volume, through a tubular reactor heated at 400 C. to500 C. under substantially atmospheric pressure. The fluoroalkene isrecovered by rectification and thereby separated from hydrogen fluorideand unreacted polyfluoroalkane and possible traces of organicby-products such as acetylenes or allenes. Traces of acetylene which maybe formed in the pyrolysis o1 1,1-di- 2. fiuoroethane, can beremoved byscrubbing with a suitable absorbent for acetylene, such as ammoniacalcuprous chloride; Alternatively, the pyrolysate may be passed through anabsorbent for hydrogen fluoride, such as soda-lime, then through anabsorbent for acetylenic by-products, and the fluoroalkene separatedfrom unreacted polyfluoroalkane by rectification.

A suitable apparatus for carrying out the process comprises a means forcontinuously metering both oxygen and the polyfluoroalkane, a flowmeteror rotometer being adapted for this purpose, a tubular reactor ofhydrogen fluoride-resistant material such as Inconel, nickel orplatinum, towers for scrubbing the pyrolysate, and a still.

The polyfluoroalkanes may be prepared by a variety of reactions. Forexample, 1,1-difluoroethane of suitable quality for the process of thisinvention may be obtained by the addition of hydrogen fluoride toacetylene by the method disclosed in application Serial No. 633,556,filed December 7, 1945, in the names of Burk, Coffman and Kalb andentitled Preparation of difiuoroethane now U. S. Patent No. 2,425,991.The method disclosed in said application comprises reacting acetylenewith hydrogen fluoride in the liquid phase in the presence of catalyticamounts of boron trifiuoride under substantially anhydrous conditions.The oxygen for use in the present invention may be commercial oxygen orair.

The following examples in which all parts are by weight unless otherwisespecified, illustrate specific embodiments of the present invention.-

Example I A mixture of 1,1-difiuoroethane and oxygen, said mixturecontaining 2% by volume of oxygen, was passed through a tubular Inconelreactor of 1%" inside diameter and a heated zone 7" long. Thetemperature of the reactor was maintained at 400 C. and the gas mixturewas passed through at a velocity such that it was in the hot zone of thereactor for about 0.5 second. The crude pyrolysate was passed throughsoda-lime to absorb hydrogen fluoride and through aqueous silver nitrateto absorb acetylene. It was condensed in a trap chilled in a DryIce/acetone cooling mixture and distilled. From parts of1,1-difluoroethane was obtained 20 parts of vinyl fluoride correspondingto a conversion of 21%. In addition, 92 parts of LI-difiuoroethane wasrecovered. Analysis of the silver nitrate solution indicated that .01part of acetylene was formed.

There was no evidence of the formation of vinyl to fluoride when1,1-diiiuoroethane containing no oxygen was passed through'a reactorheated to 400 C. at the velocity indicated.

Example II A mixture of 1,1-difiuoroethane and oxygen, said mixturecontaining 2% by volume of oxygen, was pyrolyzed in the apparatus ofExample I at 500 C. and the products were isolated as in that example.At areaction temperature of 500 C., 139 parts of 1,1-difiuoroethane wasconverted to 59 parts of vinyl fluoride, representing a 61% conversion,and 0.27 part acetylene. There was recovered 40 parts of1,1-difiuoroethane. In the absence of'added oxygen, vinyl fluoride wasnot obtained under the conditions of operation of this example.

Example In A mixture or 1,1-difiuoroethane and oxygen which contained0.5% by volume of oxygen, was

.pyrolyzed under the conditions of Example I.

Six parts of vinyl fluoride and 0.002 part of acetylene were formedvfrom 155 parts of- 1,1- difluoroethane, while 123 parts of1,1-difiuoroethane was recovered from the pyrolysate.

Example IV A mixture of 2,2-difluorobutane and oxygen, saidmixturecontaining 2% by volume of oxygen, was passed through the Inconelreactor of Example I at a temperature of 400 C. and a contact time of0.42 second. The crude pyrolysate was passed through soda-lime to absorb,hydrogen fluoride and was condensed in a trap chilled in DryIce/acetone cooling mixture. To facilitate separation and identificationof the fiuorobutene, the reaction mixture was brominated at atemperature of C. and the rewas found.

It will be understood that the above examples are merely illustrativeand that the invention broadly comprises dehydrofluorinatingpolyfiuoroalkanes containing 2 to 4 carbon atoms, in-

- clusive, with at least two fluorine atoms attached to one carbon atomand at least one hydrogen 'atom on an adjacent carbon atom, in thepresence of 0.1% to 10% by volume of oxygen at a temperature of 250 C.to 700 C.

While the specific examples show the invention as applied to1,1-difiuoroethane and 2,2-difiuorobutane, the invention is equallyapplicable to any polyfiuoroalkane containing 2 to 4 carbon atoms,inclusive, providing it is characterized by having at least two fluorineatoms attached to one car- :bon atom' and at least one hydrogen atom onan adjacent carbon atom. Other specific polyfiuoroalkanes of thisparticular type include 1,1,1- trifiuoroethane, 1,1-difluorobutane, and2,2-difiuoropropane.

A considerablevariation in the proportion of oxygen used is permissible.Based on the volume of the mixture of the polyfiuoroalkane and oxygen,concentrations of. oxy n hi h than 10% are uneconomical because thecatalytic effect does not increase appreciably above this concentrationand the poiyfluoroalkane or its dehydrofluorination product may beoxidized extensively at pyrolysis temperatures. Oxygen concentrationsbetween 1% and 4%, by volume, are especially beneficial. on the otherhand, less than 0.1% concentration of oxygen would hardly be used as theadvantages of oxygen as a catalyst are not sufilciently realized.Approximately 2% of oxygen, by volume of the mixture of oxygen and thepolyfluoroalkane, is the preferred specific concentration. Theproportion of oxygen is expressed throughout the specification andclaims with respect to the total volume of the mixture of oxygen and thepolyfiuoroalkane.

The temperature to which the mixture of gases are subjected-should bebetween 250 C. and 700 C. and, preferably, should ,be in the morelimited range of 300 C. to 600 C. At temperatures below 300 C. theconversion to fluoroalkenes becomes relatively low while above 600 C.side reactions, especially acetylene formation, tend to be excessive.Normally, a temperature of'400 C. to 500 C. will be used as this rangerepresents the most advantageous balance of the factors involved.

Pressure is not a critical factor in the reaction provided the reactantsare in the vapor state; this obviously excludes pressures so great thatthe reactants would be liquefied at the temperature employed. Thepressure may well be varied anywhere from 0.1 to atmospheres butatmospheric and subatmospheric pressures are preferred because theextent of conversion of polyfluoralkane. to fiuoroalkene increasessomewhat as the pressure under which pyrolysis is eflected decreases.

Heating of the mixture of polyfiuoralkane and oxygen, that is, thecontact time, will in all instances be relatively short and not exceedthree minutes in duration. Although it will de mi somewhat on thespecific apparatus used, 1 has been found the process may be mostsuccessfully carried out with good yields of fiuoroalkenes using acontact time from 0.25 to 2.0 seconds. A contact time as short as 0.02second may be used.

The process of this invention can be carried out very satisfactorily bypassing the mixture of oxygen and polyfluoroalkane through a reactorheated to the desired temperature. It is desirable that the gases bepassed through the reactor at a suificiently high linear velocity sothat the nature of the flow is turbulent rather than viscous. Underturbulent flow conditions the polyfiuoroalkane is heated more uniformlyand higher conversions to the desired fluoroalkene with less formationof undesired side-products may be obtained. The linear velocity at whichthe fiow becomes turbulent is a function of reactor diameter,temperature, and the nature of the specific polyfiuoroalkane and cannot,therefore, be defined precisely. Nevertheless, velocities of the orderof 0.5 to 100 meters per second are generally satisfactory. A turbulentflow is not essential, even though desirable, because good conversionsto fiuoroalkenes may be obtained within the viscous flow region,provided the contact time limits which have been set forth above, arenot exceeded.

An outstanding advantage of the present invention is that it provides apractical means of dehydrofiuorinating polyfiuoroalkanes at relativelymoderate temperatures with good yields of the desired fluoroalkenes andminimum formation of acetyleni'c compounds. The fluoroalkenes producedby the process of this invention are useful as intermediates in thesynthesis of fluoro- 0 hydrocarbons. Members of this class of organicmaterials may be polymerized or interpolymerized to form valuableplastics or resins. A particularly valuable fiuoroalkene is vinylfluoride from which polymers of excellent physical properties andunusual inertness may be prepared.

As many apparently widely diiferent embodiments of this-invention may bemade without departing from the spirit and scope thereof, it is to beunderstood that the invention is not limited to the specific embodimentsthereof except as defined in the appended claims.

I claim:

1. Process of dehydrofluorinating a polyfluoroalkane containing 2 to 4carbon atoms, inclusive, with at least two fluorine atoms attached toone carbon atom and at least one hydrogen atom on an adjacent carbonatom, which comprises heating in the vapor state a mixture of saidpolyfiuoroalkane and 0.1% to 10%, by volume of said mixture, of oxygenat a temperature of 250 C. to 700 C.

2. Process of dehydrofluorinatinga polyfluoroalkane containing 2 to 4carbon atoms, inclusive, with at least two fluorine atoms attached toone carbon atom and at least one hydrogen atomon an adjacent carbonatom, which comprises heat ing in the vapor state a mixture of saidpolyfiuoroalkane and 1% to 4%, by volume of said mixture, of oxygen at atemperature of 300 C. to 600 C.

3. Process of dehydrofluorinating a polyfluoroalkane containing 2 to 4carbon atoms, inclusive,

with at least two fluorine atoms attached to one,

carbon atom and at least one hydrogen atomon an adjacent carbon atom,which comprises heating in the vapor state a mixture of saidpolyfluoroalkane and approximately 2%, by volume of said mixture, ofoxygen at a temperature of 400 C. to 500 C.

4. Process ofdehydrofiuorinating 1,1-difluoroethane which comprisesheating in the vapor state a mixture of said 1,1-difluoroethane and 0.1%to 10%, by volurre of said mixture, of oxygen at a temperature of 250 C.to 700 C.

5. Process of dehydrofluorinating 1,1-difluoroethane which comprisesheating in the vapor state a mixture of said 1,1-difluoroethane and 1%to 4%, by volume of said mixture, of oxygen at a temperature of 300 C.to 600 C.

6. Process of dehydrofluorinating 1,1-difluoroethane which comprisesheating in the vapor state a mixture of said 1,1-difiuoroethane and Thefollowing references are of record in the file of this patent:

UNITED STATES PATENTS Name Date Mugdan et al June 26, 1945 OTHERREFERENCES Biltz, "Ber. Deut. Chem," vol. 35, 'p'ages 3524- 3523 (1902).

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